Apparatus for screening ore



Feb. 9, 1937.

C. A. GALLAGHER ETAL APPARATUS .FOR SCREENING on];

Filed May 24, 1933 3 Sheets-Sheet 1 INVENTO Char/e5 l7. Gal/a9 erATTORNEY 1937- c. A. GALLAGHER ET AL 2,070,530

APPARATUS FOR SCREENING ORE Filed May 24, 1933 3 Sheets-Sheet 2 ATTU RNRY Feb. 9, 1937- c. A. GALLAGHER :1- m. 2,

APPARATUS FOR SCREENING ORE 7.

Filed May 24, 1933 3 Sheets-Sheet 3 l VENTORS Char/e5 Gal/aye! Carl M.fll/en 7moM ' Patented Feb. 9, 193 7 PATENT [OFFICE APPARATUS FORSCREENING ORE Charles A. Gallagher and Carl M. Allen, California, Pa.,assignors to General Chemical Com- Daily, New York, N. York ApplicationMay 24,

6 Claims.

This invention is directed to apparatus for roasting metallic sulfides,and more particularly for roasting finely divided sulfide ores, todesulfurlze the same and to produce sulfur dioxide for use in themanufacture of sulfuric acid, or

for any other purpose desired.

Generally speaking, to a large extent present practice in roastingsulfide fines includes the use of mechanically operated multiple hearthconstructions, such, for example, as the well known MacDougal,Herreshofl and Wedge burners, and while such burners provide effectiveroasting, the complicated construction and operation thereof involveconsiderable initial and maintenance expense.

- As distinguished from bed roasting carried out in such burners, it hasbeen previously suggested to roast fines while in gaseous suspension, inoperations in which the fines are either ininto a roasting chamberwherein the fines encounter cross or counter-currents of oxidizingand/or suspending gas. 1

Suspension roasting is best applicable when the fines ores are in arelatively finely divided state, and this method presents the notableadvantage over the mechanical multiple hearth operation of considerablylowering the cost of production, by reason of the elimination ofcomplicated and moving parts, expensive rabbling and other apparatusinherent in the construction and operation of mechanical burners. In thepractice of suspension roasting, however, and particularly as applied toroasting metallic sulflde fines, difllculties are encountered which mustbe overcome before theoretical advantages of this type of roasting aremade practicably available. Oneof such difilculties involvesintroduction 40 of fines into the combustion chamber, and the formationof a uniform dispersion of ore particles therein. This phase of theprocess usually requires, for eflicient conduct thereof, a relativelydry ore, free from lumps and extraneous foreign material. Damp or wetore tends to clog feed pipes and injector mechanisms thus interferingwith operations in this respect. The presence of lumps in the fines,after introduction into the combustion chamber, is detrimental to theformation of a satisfactory ore dispersion in the roasting atmosphere.Furthermore, large lumps once in the roasting chamber are relativelyincompletely desulfurized, appreciable quantities of sulfide sulfurbeing retained therein, thus in- Jected into a roasting chamber insuspension il the oxidizing gas, or are showered downwardly'" Y., acorporation of New 1033. Serial No. 572,550

(on. a-25'n creasing the sulfur content of the cinder, and lowering theefiiciency of roasting.

To avoid such difiiculties, it is desirable to feed into the furnace,dry, or dry and preheated ore free from lumps. This requires screening,and screening should preferably be effected after drying, as when dampore is screened, the damp material tends to clog the screens. Moreover,if the fines are screened prior to drying, lumps which may form duringdrying are injected, as such, into the combustion chamber. Hence, forefficient operation of processes for roasting metallic sulfides ingaseous suspension, it is preferable to dry or dry and preheat thefines, and subsequently screen the same before charging into thefurnace.

Prior screening devices, while effective in a general way, have beenmore or less objectionable because of attention required to keep suchdevices in operation, to prevent clogging with lumps and foreignmaterial and to prevent overflow of fine material therefrom. It has alsobeen proposed to dry fines ores prior to injection into the combustionchamber by causing the ore to pass in heat exchange relation over thetop and sides of the combustion chamber thus utilizing radiated andconducted heat to dry the ore.

It is one of the principal objects of the invention to providesatisfactory mechanism for screening ore before feeding the same intothe furnace injectors. Another object is to provide a screening devicewhich is automatic in operation and which does not require attention ofan operator. The invention contemplates apparatus for preliminary dryingor drying and preheating of the fines, and for screening such fines,prior to injection into the roasting chamber.

With respect to the improved screening mechanism some of the importantfeatures thereof comprise provision for regulation of the amount offines fed to the screen surface to that withdrawn from the subjacentscreenhopper conducting the screened fines into the injector mechanism;continuous withdrawal from the screen of oversized and foreign material;an auxiliary overflow screen and hopper, positioned beyond the mainscreen surface and associated hopper, which overflow screen and hopperact to prevent discharge from the screening device of undersizedmaterial, thus preventing loss of the latter; and an arrangement bywhich material is ,discharged from the overflow hopper into the finesinjector faster than from the main hopper, so that the overflow hoppercannot become filled and cease to function in the intended capacity.

The objects and advantages of the invention may be more clearlyunderstood from a consideration of the following description taken inconnection with the accompanying drawings in which Fig. 1 is averticalsection of a burner in connection with which the improvements of"the invention may be advantageously employed;

Fig. 2 is an enlarged, vertical section of a fines oreinjectormechanism;

Fig. 3 is a vertical section of another type of burner illustratinganother adaptation of the screening device of the invention:

Fig. 4 is an enlarged, vertical section of the improved screeningdevice, showing a portion of a burner such as illustrated in Fig. 3;

Fig. 5 is a plan view of the hopper of the screening device;

Fig. 6 is an elevation of the hopper taken substantially on the line 66of Fig. 4;

Fig. '7 is a development, on a reduced scale, of an ore drying andheating jacket surrounding the walls of the burner of Fig. 1, and

Figs. 8 and 9 are sections of a hopper screen agitator.

Referring particularly to Fig. 1 of the drawings, numeral l0 designatesgenerally a shaft bur er comprising a shell ll constructed of suitablefractory material, such as firebrick, and efining a roasting orcombustion chamber l2 0 cylindrical cross-section. Surrounding the shellII is a steel casing I3, acting as a protective reinforcement for thefurnace. The upper end of the combustion chamber is closed off by a.crown 15, the top side of which forms a drying and preheating hearth IT.The cylindrical shell II and casing l3 project upwardly beyond thecrown, and carrry a framework H! which in turn supports ore feeding andrabbling mechanism for hearth II.

The surface of hearth i1 is slightly cone-shaped and slopes downwardlytoward the shell of the burner. Lying above the hearth are rabble armsl9, having downwardly projecting plows 20 pitched to work sulfide finesgradually toward the circumference of the drying hearth. Arms I9 arerotated by a motor 22 through shaft 23, supported in bearings so as tomaintain the lower ends of the plows 20 properly spaced with respect tothe surface of the drying hearth H. An ore bin 24, mounted on frameworkl8, discharges ore onto a platform 25 from which fines areintermittently dropped onto approximately the center of hearth H by asweep 21 rotating with shaft 23.

Surrounding the major portion of easing l3 are a plurality of louvers 30which with casing l3 form an ore drying and preheating jacket designatedgenerally by reference numeral 32, enveloping a substantial portion ofthe vertical walls of the burner. Cut in shell ll, near the upper end,are downwardly sloping ore supply passages 34 through which ore ispassed from the drying hearth I! into the topmost louver 35 constitutingthe upper edge of jacket 32. The drawings shows, in section, twopassages 34. Preferably, several such conduits may pass through the topof shell II at spaced points about the circumference of the burner sothat, on rotation of the rabble arms l9, fines are gradually fed throughthe openings 34 into the top of the drying jacket.

In the embodiment of the apparatus shown in Fig. l the jacket 32 isprovided at the bottom with three outlets, indicated at 38, throughwhich fines are fed into the subjacent screener, designated by referencenumeral 40. As shown in section at the left of Fig. 1 immediately abovescreener 48, and also in elevation in Fig. 7, sloping side 42 oflowermost louver 43 is of greater length than the sides of other louversso as to provide small hoppers, terminating in discharge openings 38.The bottom of jacket 32 is formed by sloping edges 45 which intersectlower louvers 43 several of the intermediate louvers 30, and areattached to steel casing l3. A development ofthe louvers is shown inFig. 7. Edges 45 of the jacket are sloped at an angle greater than theangle of repose of the sulfide fines, so that the latter run freelytoward outlets 38 through which the material is fed into the screener40. The louvers are supported by ledge-like projections 48 of verticals49 which in turn may be suspended from framework l8. The louvers restloosely on the projections, this arrangement permitting repair orreplacement.

The construction of the screener is best illustrated in Figs. 4, 5 and6. The screener 48 includes principally a funnel-like casing 58,preferably having the shape of an isosceles trapezoid in horizontalcross-section, and terminating at the bottom in an outlet pipe 5| forfeeding material into an observation funnel 52 on the top end of aninjector feed pipe 53 havinga control valve 54. Projecting upwardly fromthe top edges 56 of the sloping sides 51 of casing 58 are vertical sides59 converging in a direction away from the roaster shell, and forming apassage 60 through which oversized material is discharged, over a lip6|, into a collecting pipe 62. Set into casing 50 is a transverse baiile63 attached at the side edges to sides 51 of casing 50. The bailieslopes downwardly toward the burner shell as shown in section in Fig. 4and in plan in Fig. 5.

It is to be observed the lower edge 64 of bafile 63 crosses the axis ofoutlet pipe 5| so that the passage 65 between edge 64 and the back 61 ofcasing 50 is appreciably more restricted than the passage 10 between thelower edge of baffle 63 and the inner face of front 1| of the casing 50.Baiiie 63 forms with back 61 what may be considered a primary hopper l3,and forms with front ii an overfiow hopper 14. The inner top edge ofcasing 50 may be made arcuate as at 12 to permit the hopper to fitsnugly against shell I 3.

A screen 11 covers hoppers l3 and I4, and is supported in the positionshown in Fig. 4 by rear edge 12, the top edge of the baffle 83, frontlip iii of casing 50, and by the sloping sides 5'! thereof.

Referring again particularly to Figs. 1 and 2, the injectors forintroducing fines into the furnace comprise a principally metallic pipesection 80, the inner end of which is set into the shell of the burnerat the lower end of the combustion chamber, pipe providing an ore inlet8|. Tube 80 is held securely in position by a circular plate 82 and aflange 83 bolted to steel shell l3. In the preferred construction, theaxis of tube 80 is directed upwardly at a relatively sharp angle, forexample about 75 to the horizontal.

As shown in Fig. 2, the valve-controlled pipe 53 projects into the upperside of tube 80. A gas inlet pipe or jet 85 passes through end plate 86and terminates beneath the discharge end of pipe 53. Admission of air orother gas to jet 85 is controlled by a valve 81. A plug 89 affords meansfor cleaning out conduit 8| should the same become stopped up.

Surrounding the burner near the injectors is a bustile 90 for supplyinggas to injector jets 85, the bustle being connected to agas supplythrough a pipe 3| controlled by valve 92. oxidizing gas to support theroasting operation is introduced into the shell I2 throughcircumferentially spaced ports 33, positioned adjacent the oreinjectors, and connected through short pipe sections not shown with abustle 04. The bustle 94 may be provided with an inlet pipe connection,having a control valve, either open to the atmosphere or connected to ablower when it is desired to operate the burner under positive pressure.

The bottom of the combustion chamber is formed by a hopper-shaped hearth05 pitched at an angle preferably slightly less than the angle of reposeof the oxide cinder, and terminating in an outlet 96 through whichcinder is continuously discharged into a conveyor. A gas main 31 forwithdrawing gaseous products of combustion from the chamber I2 opensinto the latter just beneath the crown I5.

In the specific embodiment of the invention described, the height of thecombustion chamber of the furnace, exclusive of the hopper-shapedbottom, is about equal to the diameter. While such particular proportionof the combustion chamber is preferred when carrying out the roastingoperation per se in a burner of the general type illustrated in thedrawings, it is to be understood that the diameter of the chamber may insome instances advantageously exceed the height by a substantial amount,and may also be less than the vertical dimension. Preferably thediameter of the chamber is not less than the height. It will beunderstood, however, that the apparatus relative to the means for dryingor drying and preheating the fines, prior to introduction into thecombustion chamber and relative to the improved screening device may beemployed in connection with any kind of roasting chamber.

Fig. 3 represents a different type of burner in connection with whichthe screener 40 may be used to advantage. In this apparatus, the generalconstruction thereof is substantially the same as already described inconnection with Figs. 1 and 2. However, in Fig. 3, the ore injectors I00are positioned a relatively short distance beneath the crown I5 so thatthe finely divided ore is fed into the top of the combustion chamber I2.Screener 40 feeds ore from supply passages IOI through inlet pipes I02into the injectors I00. When employing this type of apparatus, it ispreferable to position injectors I00 relatively close to the undersideof crown I5, and hence desirable to shorten the distance between theinjectors and the screeners as much as possible to conserve head room.Accordingly, angle A (Fig. 4), representing the angle between ,thesurface of screen 11 and the horizontal may be less than would otherwisebe the case. In this situation, it may be advantageous to intermittentlyagitate or shake screener 40 in order to work coarse material over thesurfacee of the screen, and through discharge opening 60. For thispurpose, there may be provided a knocker I05 rotatably journaled in abearing I06 carried in the top end of the shell II. The knocker I05includes an arm I01 the lower end of which projects into the path ofrabble arms I0, and a second arm I08 normally resting against one side59 of screener 40.

Figs. 8 and 9 show a modified construction in which the screen isvibrated electrically. Screen I25, having outwardly projecting sidesI26, is pivoted at the rear edge at I21 to the back 61 of casing 50, theopposite edge of the screen exmaterial thereon is carried by bracketthrough nut 35 and vibrator rod I36. The up- .edges of sides I28 of thecasing is a bracket I30 carrying a vibrator I3I the casing I32 of whichmay be rigidly connected to support I30 as at I33. In the center of thebracket I30 is a circular opening I35 through which passes a screenvibrating and supporting rod I36. Attached to the upper end of rod I36is a vibrator disk I31 spaced from fixed core I38 of the vibrator.Underneath the disc I31 is a nut I40 attached to rod I36 and arranged tobear on the upper side of bracket I30 when the core I38 is demagnetized.The lower end of rod I36 is connected to screen I25 by set nuts I4Iwhich are adjusted so that the outer end of the screen is held spacedslightly above the upper surface of lip 6i of the casing. When the coreI30 of the vibrator is demagnetized, the weight of screen I25 and theI30 per end of core I38 is threaded into the top of the casing I32 andis provided with a lock-nut I44 by means of which the air gap betweendisc I31 and the lower end of core I36 may be adjusted. Alternatingcurrent is connected to coil I45 by wires not shown.

The invention is applicable to the roasting of metallic sulfide finessuch as iron pyrites, pyrrhotite, zinc sulfide or arsenopyrite, but forconvenience, and by way of illustration only, the operation of theapparatus may be described in connection with the roasting of ironpyrites.

A supply of relatively finely divided pyrites is maintained in the bin24 by suitable conveyor or elevator mechanism, not shown. roasting isbegun, combustion chamber I2 is preheated to temperatures above theignition point i the particular ore to be roasted, as by the use of oilburners inserted through conveniently located workholcs, not shown. Whenthe desired degree of preheat is obtained in the combustion.,chamber,the motor 22 is started, and rabble'arms I9 and sweep 21 may be rotatedat a rate of say one revolution in two minutes. Fines run continuouslyout of the bin 24 onto platform 25, and, on each revolution of shaft 23,a regulated quantity of ore is swept off the platform to approximamlythe center of hearth I1.

During rotation of rabble arms IS, the fines are gradually worked acrossthe heated surface of hearth I1 into passages 34. The dry or partly dryore then runs into the louver 35 and the top of jacket 32. The louvers30 are spaced sufficiently close so that material, while running fromone louver into the one immediately below does not overfiow the upperhorizontal edge of the lower louver, that is, the surface of materialbetween the lower edge of a louver and the upper edge of a subjacentlouver, for example, as indicated at H5, is substantially at the angleof repose. The numerous exposed surfaces of the fines between theseveral louvers permit intermittent exposure to the atmosphere of atleast some of the fines and thereby provides for the ready escape ofwater vapor formed during drying or drying and preheating of the fines.

When roasting is under way, hearth I1, and the vertical walls of thecombustion chamber become relatively highly heated, and sulfide finesare dried and preheated to some extent during movement over the dryinghearth I1 and through jacket 32 surrounding preferably at least a majorportion of the burner shell. This operation blankets the top and sidesof the burner with ore which acts as a heat insulator, retaining heat inthe combustion chamber, and employing heat Which'is transmitted throughshell II, to the ore, to dry and heat thelatter. This preliminaryheating serves to dry theflnes, thus facilitating the formation of moresatisfactory dispersion of ore in the combustion chamber, and at thesame time preheats the fines to temperatures, say 300 to 400 F., so asto promote ignition shortly after introduction into the combustionchamber.

The fines dried, or dried and preheated, on

passage'thru jacket 32 are directed-by sloping edges 45 toward outlets38 through which the material is fed to' the subjacent screeners 40, be-

fore injection into the combustion chamber. The

' as to tend to continuously feed fines ontoscreen TI. The slope of thelatter, represented by angle A, is preferably such that oversizematerial is gradually worked over the surface of the screen anddischarged through opening 60. Generally speaking, the upper horizontaledge II 6 of baffle 63 is spaced with respect to the burner shell I3 sothat the remote edge or toe I I! of the mound II8 of material on top, ofthe screen I1 lies about at the position of horizontal edge I I6 of thebaffle 63, and so that the upper surface of the mound II8 assumes theangle of repose. According to the nature and particular size of theparticular fines, the position of the toe III may be adjusted byvertically slidable gate I20 so that toe III lies, substantially abovethe upper edge of the baffle 63. By this arrangement, there ismaintained on the screen above hopper I3 a given amount of materialwhich acts in turn to lute off passage IM and control the amount of orefed therefrom to the screen.

The bulk of the material passes through screen 11 and into the primaryhopper 13 from which the fines are withdrawn through passage 65, andpipe 5I into observation funnel 52 on the top of feed pipe 53. From aconsideration of Fig. 4, it will be seen that if material less thanscreen size works down over the surface of the screen and passes theupper edge of baiiie 63, such undersized material will pass through thelower end of the screen and into the overflow hopper I4. Thisconstruction prevents discharge over lip 6| of material of less thanscreen size. It will be observed lower edge 64 of bafiie 63 crosses theaxis of outlet pipe 5I so that the passage I is larger than passage 66.For this reason, screened material will feed out of hopper 14 into pipeI at a rate faster than out of hopper I3. Accordingly, materialcollecting in hopper I4 is quickly fed out through opening I0 into thefunnel 52 thus insuring an empty 'or at least partially empty terial offscreen, I1 and out through opening '80.

In the operation of the screener 40, only the equivalent of theorewithdrawn through pipe 53 can be fed to screen I1. As the same amount ofmaterial is fed to the screen as is withdrawn continuously through pipe53, segregation of relatively fine and coarse undersize material isprevented. The feed conduits to the screen, such as passages I0 I, aresealed by material on the screen, and overflow of the screen is avoided.Coarse oversize finaterial is discharged without blocking the hopper,and material of less than screen size which might be discharged alongwith the oversized material is caught in overflow hopper I4, anddirected into the ore injector through pipe 53.

The operation of the screener of Fig. 1 is substantially the same asdescribed in connection with Fig. 4. In Fig. 1, it will be understoodthe construction .is sothat the horizontal spacing between the upperedge of battle 63 and the lower edge of bottom louver 43 is such that aline drawn between the two represents with the horizontal approximatelythe angle of repose of the particular material being used for roasting.If desired, a small adjustable gate corresponding with gate I20 of Fig.4 may be attached to the lower edge of louver 42 so as to permithorizontal adjustment of the position of the toe of fines on the surfaceof screen 11 relative to the upper edge II6 of bafile 63.

In the operation of the modified apparatus shown in Figs. 8- and 9, amound of material II8 is maintained on screen I 25 as already describedin connection with Fig. 4. In the construction of Figs. 8 and 9,however, provision is made for continuously vibrating screen I25. Whencoil I45 of vibrator I3I is connected to a source of alternatingcurrent, it will be understood core I33 is alternately magnetized anddemagnetized and disc I3'I carrying screen I25 is raised to engage thelower end of core I38, and then drops back to the position shown in Fig.8. It will be understood this construction continuously vibrates screenI25 about pivot I 21 thus insuring continuous feed to the hoppers 13 andI4, and avoid ing clogging of the screen. This construction may beemployed to advantage where the angle formed by the surface of screenI25 and the horizontal is not sharp enough to effect prompt discharge ofoversized material from the screen.

Valves 54 in feed pipes 53 are adjusted so that a substantially steadystream of fines runs into feed conduits 8I. into the combustion chamberI2 by means of air,

. steam, or other gas not adversely afiecting oxidation of the sulfide.It is preferred to employ air for this purpose, and when using thelatter, air may be introduced into the lower ends of conduits 8| throughvalve-controlled jets 85 at pressures of about 5 pounds per square inch.When starting up roasting operations, it will be understood thecombustion chamber I2 is preheated to above ignition temperature of thefines by oil burners or other convenient means not shown.

The axes of inlet conduits 8 I, Fig. 1, the amount of fines fed into theinjectors through pipes 53, and the air pressure in jets 85, regulatedby valves 81, are all adjusted with respect to the particular size ofthe roasting chamber so that the ore particles from each injector risethrough the combustion chamber to an elevation just below the undersignof crown I5. Ore particles then fall to hearth 35, and while passingthrough an atmosphere increasingly rich in oxygen, oxi- The fines arethen injected dation of the fines is completed. It will be understood inthe embodiment of the invention shown in Fig. 1, the major portion ofthe air for roasting is drawn into the combustion chamber through ports93 by a suction fan in the gas outlet conduit 91 through which thegaseous prodnets of roasting are withdrawn. The roasting operation perse is no part of the present invention, and hence need not be furtherdetailed.

The operation of the apparatus of the invention when used in connectionwith the burner illustrated in Fig. 3 is thought to be apparent from thedescription already given relative to the enlarged section shown in Fig.4. However, when employing the apparatus of Fig. 3, it will beunderstood the fines are injected into the upper,end

of the combustion chamber and fall thro h-a rising current of air orother oxidizing gas ntroduced into the bottom of the combustion zone asin the apparatus of Fig. 1.

The sulfur dioxide gases produced may be used, for example, in themanufacture of sulfuric acid. The sulfur dioxide content of the burnergas may be regulated as desired by adjusting the amount of air fed -intothe combustion chamber as is known by those skilled in the art. Gasescontaining 10-13% sulfur dioxide may be readily made by the presentprocess.

We claim:

1. Apparatus for screening material comprising a casing, a slopingscreen covering the casing, an outlet at the bottom of the casing, abafile in the casing forming primary and overflow hoppers, the upperedge of the bafiie lying against the screen and the lower edge of thebaffle being positioned, with reference to the axis of the casingoutlet, toward the material feed end of the screen, means for feedingmaterial to the higher end of the screen, means for dischargingoversized material over the lower end of the screen, and means forwithdrawing material from the bottom of the casing.

2. Apparatus for screening material comprising a casing, a slopingscreen covering the casing, an outlet at the bottom of the casing, abaffle in the casing forming primary and overflow hoppers, the bafflesloping downwardly in a direction toward the high end of the screen, theupper edge of the baffle lying against the screen and thelower edge ofthe baffle being positioned, with reference to the axis of the outletconduit, toward the material feed end of the screen, means for feedingmaterial to the higher end of the screen so as to maintain the toe ofthe mound of material thereon adjacent the edge of the baffle, means fordischarging oversized material over the lower end of the screen, andmeans for withdrawing material from the bottom of the casing. I

3. Apparatus for screening material comprising a screen, a casingprovided with a discharge opening beneath the screen, abafliein thecasing forming with the walls of the casing a primary hopper and anoverflow hopper, each of said hoppers having discharge openings formedby the lower edge of the bafile and the walls of the casingcommunicating with the discharge opening of the casing, the dischargeopening of the overflow hopper being larger than the discharge openingof the primary hopper, means for feeding material to the screen abovethe primary hopper, means for discharging oversize material from the endof the screen beyond the overflow hopper and means for withdrawingmaterial from the discharge opening of the casing.

4. Apparatus for screening material comprising a casing having sidewalls, the said walls having upper edges forming an angle with thehorizontal, a screen disposed above the casing at the same angle as theupper edges of the side walls of the casing, a baflie in the casingdividing the easing into a primary hopper having as side walls thehigher portions of the side walls of the casing and an overflow hopperhaving as side walls the lower portion of the side walls of the casing,means for introducing material to be screened onto the screen above theprimary hopper, and means for withdrawing oversized material from thescreen beyond the overflow hopper, the angle of the upper edges of theside walls of the casing and the screen being such as to maintain amound of the material on the screen so that the toe of the mound isadjacent the upper edge of the bafile and the upper surface of the moundof material is substantially at the angle of repose of the material. v

5. Apparatus for screening material comprising a casing, a screencovering the casing, an outlet at the bottom of the casing forwithdrawing material therefrom, a baflle in the casing forming primaryand overflow hoppers, the upper edge of the baflle lying against thescreen and the lower edge of the bafile being positioned, with referenceto the axis of the casing outlet, toward the outer end of that portionof the screen covering the primary hopper, means for feeding material tosaid outer end of the screen, and means for discharging oversizedmaterial over the opposite end of the screen.

6. Apparatus for screening material comprising a casing, a screencovering the casing, an outlet at the bottom of the casing, forwithdrawing material therefrom, a baffie in the casing forming primaryand overflow hoppers, the upper edge of the baffle lying against thescreen and the lower edge of the baffle being positioned, with referenceto the axis of the casing outlet, toward the outer end of that portionof the screen covering the primary hopper, means for feeding material tosaid screen so as to maintain the toe of the resulting mound of materialon the screen covering the primary hopper not substantially beyond theedge of the baflie, and means for discharging oversized material overthe opposite end of the screen.

CHARLES A. GALLAGHER. CARL M. ALLEN.

